This invention relates to methods and apparatuses for measuring the average velocity of a flow stream.
In one class of velocity measuring apparatuses, ultrasonic signals are transmitted through the flow stream of a fluid and the reflections received from reflective portions of the fluid are sensed. The Doppler shift in frequency between the transmitted signal and the received signal is used to determine the average velocity of the fluid.
In this class of average-velocity measuring apparatuses, the waveform for the combined Doppler shifts in frequency of reflected ultrasonic sound represents an average velocity of the flow stream because the Doppler frequency shift for each portion of the flow stream is proportional to the velocity of that portion, the amplitude of the sum of the reflected signals for each different frequency shift represents the volume of the fluid flowing with that velocity, and thus, the sum of the received signals is a waveform combining the amount of each different velocity portion of the fluid. Each different velocity portion of the fluid stream contributes to a component of the waveform and its component is proportional to the contribution from each other different velocity portion so that the amplitude for each corresponding frequency shift represents the proportionate amount of fluid flowing with that velocity.
The signals that are incident on reflecting portions of the flow stream near the transmitting transducer or transducers have a higher amplitude than those incident on reflecting portions of the flow stream more remote from the transmitting transducer. The difference in amplitude or intensity is caused by the distribution of energy through a solid angle as it moves from the transmitting transducer to the reflecting portion of the fluid. However, the energy incident on the remote portions impacts on a larger proportion of the fluid at each velocity at more remote distances than at close distances for reflection.
It is believed that this class of average-velocity measuring apparatuses relies on the nature of the flow stream and the intensity of the transmitted signal being such that an approximate compromise can be reached in which the attenuation and reduction intensity with distance is balanced by the increased area from which signals are reflected. This attenuation is caused by the wider distribution of the energy of the transmitted signal and the increased attenuation of the reflected signal over the longer distances. This balance causes the energy transmitted to areas at a distance before being reflected to result in a sensed signal the same as if the entire reflected energy had been reflected from the same plane in the cross-section of the flow stream so that the signal is representative of an average velocity of that cross-section.
Because the received signals mainly represent those sound waves that travel a straight path to the reflective portions of the flow path and are reflected in a straight path to the receiving transducer, the received signals do not include representative amounts of sound waves that are reflected at an obtuse angle such as by glancing off at an angle from a portion of the flow path nor do they include representative amounts of reflected sound waves from certain sides or low portions of the flow path. Thus, the final waveform may actually not include sound waves reflected from the entire cross-section because the transmitted waves miss some portions of the flow stream and some of the reflected waves do not impact directly on the receiving transducer. However, the final waveform must represent the total cross-section.
To cause the final waveform to represent the total cross-section, even though the receiving transducer does not receive a representative amount of sound waves from every portion of the flow path, a representative portion of the flow stream should be selected for measurement of average velocity in this class of average-velocity measuring instrument. This representative portion can be sensed by selecting the angle of the transducers to cut proportional amounts of each velocity of flow.
One prior art velocity measuring system of this class was manufactured and sold by a corporation called Montedoro-Whitney Corporation. That prior art apparatus received different frequency signals in the expected range on a transducer and filtered a set of frequencies which were then weighed and averaged.
This type of measuring apparatus has several disadvantages, such as for example: (1) the range of signals of interest shifts as the velocity of the flow stream shifts, resulting in some inaccuracies due to the selection of a less desirable set of frequencies to be examined; (2) the on-line measurements of a limited number of ranges of frequencies accomplished by that system results in some lack of precision; and (3) because of the lack of precision, an empirically determined velocity coefficient is desirable at most locations to correct the measurement.